1. Field of the Invention
The present invention generally relates to electro-optical readers, such as laser scanners and, more particularly, to improved reader performance in ambient light of high intensity during reading of indicia, such as bar code symbols.
2. Description of the Related Art
Bar code readers are known in the prior art for reading various symbologies such as Universal Product Code (UPC) bar code symbols appearing on a label, or on the surfaces of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like.
Readers of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, assigned to the same assignee as the instant application, and may employ a portable laser scanning device held by a user, which is configured to allow the user to aim the device and, more particularly, a scanning laser light beam, at a targeted symbol to be read. In moving laser beam readers known in the art, the laser light beam is focused by a lens or other optical components along a light path as a beam spot on a target that includes the bar code symbol. The moving-beam reader operates by repetitively scanning the beam spot in a scan pattern across the symbol by means of motion of a scanning component, such as a moving scan mirror placed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line, or a series of scan lines, or another pattern, across the symbol, or scan a field of view of the reader, or both.
Bar code readers also include a sensor or photodetector which detects light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The light is detected and converted into an electrical signal. Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector is converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented. Such signal processors are disclosed in U.S. Pat. No. 5,734,153, assigned to the same assignee as the instant application.
Bar code readers are required to work under variable ambient lighting conditions, including indoor office lighting and outdoor sunlight, both indoor and outdoor lighting ranging from dim to bright. A combination of optical and electrical measures is employed to prevent bright ambient light from overwhelming the reader and preventing a successful decoding and reading of a symbol to be read. Such measures can be optimized for indoor or outdoor lighting at anticipated light intensity levels, but not for both. Thus, performance is sacrificed when a reader is required to work at non-optimized light intensity levels.
One category of reader is termed retro-reflective or retro-collective and refers to the light collection optical system in which a field of view is scanned along with the laser beam. Typically, the scan mirror used to transmit the laser beam to the symbol also collects the light scattered from the symbol and directs the collected light to the sensor. This allows the field of view to be relatively small since it only needs to be large enough to see the focused laser beam spot plus accommodate manufacturing tolerances that tend to move the beam spot out of the center of the field of view.
The scan mirror in a retro-reflective reader needs to be relatively large, since the surface area of the scan mirror determines how much scattered light can be collected. However, a large scan mirror requires more electrical energy to be oscillated during scanning, can cause undesirable vibration when oscillated rapidly, and requires more protection from external shock forces. Even though these factors are undesirable for a handheld reader, most readers employ a retro-reflective system, primarily because such readers are relatively insensitive to different levels of ambient light that the small field of view affords.
Another category of reader is termed non-retro-reflective or non-retro-collective and refers to the light collection optical system in which the field of view is stationary. Typically, the scan mirror is only used to transmit the beam to the symbol, while a separate optical system having a wide, static field of view is employed to collect scattered light. The beam spot is swept in a scan pattern across the symbol, and the field of view must be large enough to allow the sensor to see the entire scan pattern, which may be a single line or a plurality of lines. Hence, the field of view of a non-retro-reflective system is several times larger than in a retro-reflective system, with a proportionate increase in the amount of ambient light collected along with light scattered from the symbol. Hence, it has been difficult to employ a non-retro-reflective system that works well in all ambient lighting conditions.
However, there are clear advantages to using a non-retro-reflective system. The scan mirror can be made very small since it only needs to accommodate the focused beam spot. The small scan mirror is much easier to oscillate without consuming large amounts of electrical power, creates less vibration even at high rates of oscillation, is much easier to shock-proof to prevent damage from external shock forces, and is less expensive as compared to the larger scan mirrors of the retro-reflective systems.
Clearly, it would be advantageous, especially in the case of handheld readers, to take advantage of the benefits of a non-retro-reflective system if the ambient light issues that result from its inherently large field of view could be reduced or eliminated. One step in this direction is to reduce the field of view in a non-retro-reflective system by using a lens to control the vertical dimension of the field of view, and by using the reader's housing to limit the horizontal dimension of the field of view. This enables the reader to work reliably under all anticipated, indoor, artificially illuminated lighting conditions, but there are still compromises that must be made if the reader is to work in direct sunlight, which can be over fifty times brighter than the brightest indoor lighting.
In a typical retro-reflective or non-retro-reflective reader, collection optics collect the laser light scattered from the symbol and concentrate the collected light onto a photodiode acting as the sensor. The collection optics also unavoidably collects ambient light and concentrates it on the photodiode. The photodiode generates an electrical current proportional to the brightness of the total collected light. The current is applied to an input of a transimpedance amplifier operative for generating an output voltage proportional to the current and, in turn, proportional to the brightness of the total collected light. The output voltage increases or decreases in dependence on increases or decreases in the collected light intensity. If the collected light intensity is bright enough, the output voltage will go so high that the amplifier is incapable of going any higher. This is called saturation and, when this happens, the reader will not function because the data signal derived from the symbol will be lost.
For this reason, standard practice, when designing readers that must work in bright sunlight, has been to decrease the gain of the transimpedance amplifier to the point where it will not be driven into saturation even in sunlight. As a result, non-retro-reflective readers, which have large fields of view and collect a lot of ambient light, have had to be designed with lower transimpedance amplifier gains than those in retro-reflective readers whose fields of view are smaller and therefore collect less ambient light. The gain of these transimpedance amplifiers is controlled by a feedback resistor. Reducing the resistance of the feedback resistor reduces the gain, and vice versa.
Unfortunately, reducing the resistance of the feedback resistor reduces the data signal faster than the noise signal such that the signal-to-noise ratio of the output signal of the amplifier becomes worse as the gain is reduced. Since non-retro-reflective readers needed to have lower gains (to prevent saturation) than retro-reflective readers, the non-retro-reflective readers have had poorer signal-to-noise ratios, which degrades their performance even when operating under indoor lighting conditions. Thus, the superior signal-to-noise ratio of retro-reflective readers and the concomitant extended working range are the main reasons for their popularity.
The prior art has proposed in FIGS. 9–11 of U.S. Pat. No. 5,923,021 preamplifier circuits for processing the output signal from a photodiode. These circuits include components that reduce the resistance of the feedback resistor, thereby reducing the gain and worsening the signal-to-noise ratio, or that introduce shot or white noise from bipolar transistors and diodes which again worsen the signal-to-noise ratio. Hence, such preamplifier circuits are unsatisfactory for enabling the reader to perform reliably under all lighting conditions.